Evgen_Prozhyrko/iStock(NEW YORK) — A 26-year-old woman on the job for six months.

A father of two with 24 years of experience in law enforcement.

These are just two of the four police officers shot and killed in the United States this week — part of what one expert calls a disturbing “multi-year” trend of violence toward police.

June 23: Wellston, Missouri

In Wellston, Missouri, officer Michael Langsdorf from the North County Police Cooperative was shot and killed on Sunday night in what police described as an execution.

Langsdorf was responding to a report of a person trying to cash a bad check, and when he confronted the suspect, a struggle broke out between the two on the floor, said North County Police assistant chief Ron Martin.

The suspect pulled a gun from his waistband and hit the officer on the side of the head several times, causing Langsdorf to lose his hold, Martin said.

The suspect then got up and stood over Langdorf who was on the ground, facedown; the suspect pointed the gun at the back of Langdorf’s head and fired one shot, striking him in the neck, Martin said.

The suspected gunman has been arrested, police said.

Martin said, who knew Langdorf, was overcome with emotion when asked about the attack.

Langsdorf, 40, leaves behind two children, his parents and his fiancé, said police.

Langsdorf had 17 years of police experience and joined the North County Police Cooperative this April.

“There’s no such thing as a routine call,” Martin said. “This is the danger that our police officers in this community face every day.”

June 20: Mission, Texas

Cpl. Jose Espericueta of the Mission, Texas, Police Department was shot and killed on Thursday, June 20, authorities said.

Espericueta, a 13-year veteran, leaves behind a wife and two children, police said.

The shooting unfolded when a woman waved down an officer Thursday to say her son had fired shots at her car, police said.

Espericueta responded and tried to make contact with the suspect, who then ran away from him, police said. As the suspect ran, he turned around and began firing, police said, and he exchanged gunfire with Espericueta and other officers.

It was the first time the town saw the loss of a police officer in the line of duty since 1978, police said.

A suspect was taken into custody.

June 19: Sacramento, California

Tara O’Sullivan, a 26-year-old Sacramento police officer who graduated from the police academy in December and was hired in January, was shot and killed on June 19, authorities said.

O’Sullivan, who had partnered with a training officer and was accompanied by others from the department, had responded to a domestic dispute and was standing by while a woman gathered some of her belongings, police said. The young officer was shot in an ambush attack, police said.

Multiple officers fired back in the intense firefight, police said. About 50 minutes after O’Sullivan was shot, officers rushed in with an armored car to rescue her and take her to a hospital, according to police.

O’Sullivan was a member of Sacramento State’s Law Enforcement Candidate Scholars’ Program, which “allows young men and women to go directly into the academy to serve,” Sacramento State President Robert Nelsen said.

“We lost a hero. We lost a leader,” Nelsen said at a news conference on Thursday. “We will never forget her… and we will aspire to be as good as she is.”

June 17: Racine, Wisconsin

Officer John Hetland, a 24-year veteran of the Racine, Wisconsin, Police Department, had worked the day shift on Monday, June 17. He was off-duty and at a bar that night when he saw an armed robbery unfolding, police said. He tried to intervene and was shot, police said.

Hetland is survived by two children, Racine Mayor Cory Mason said.

“I just really can’t express how deeply we feel the loss of this officer,” the mayor told reporters last week. “It’s been more than 40 decades since we’ve had a loss in this city.”

Police are still searching for the gunman; the Kenosha County Sheriff’s Department on Sunday released three updated photos of the suspect. Authorities also ask residents to check their security cameras from that night and contact police if they notice anything suspicious.

‘Part of a multi-year trend’

Beyond the four police officers shot and killed this week, a Nebraska State Patrol trooper died in a car crash on Thursday.

Monday also marks the funeral for Los Angeles County Sheriff’s deputy Joseph Gilbert Solano who was shot while off-duty at a Jack in the Box on June 10. Solano, 50, died two days later.

But officer deaths overall, as well as officer deaths by gunfire, are down this year from last year, according to the National Law Enforcement Memorial Fund. Twenty-five officers have been shot dead this year, compared with 31 deaths at this time last year.

However, John Cohen, a former acting undersecretary at the Department of Homeland Security (DHS) and current ABC News contributor, called the slayings “part of a multi-year trend of increased acts of violence directed at law enforcement.”

“While the numbers may be less when compared to the same time period last year, if you look at it on a multi-year basis we’ve seen a significant level of violence directed toward law enforcement officers,” he said. “It’s an issue that has police chiefs and rank-and-file police officers very concerned.”

This causes officers to fear for their safety, which may impact their mental health and make them more forceful and aggressive while interacting with the public, Cohen said.

“It’s not just simply a matter of police officers feeling more threatened,” he said. “They’re much more likely to be wary when responding to a call… they may be more willing to escalate to the use of force when they perceive that they may be threatened. That may result in a violent reaction by the person they’re contacting.”

Cohen calls the “multi-year trend” of violence toward police as “a reflection of the broader levels of anger and violent behavior that’s becoming all too common across our society.”

“These increased acts of violence against police come at a time where we’re also seeing an increase in targeted attacks and hate crimes,” Cohen said. “We’re a society that has become increasingly polarized, increasingly angry and increasingly willing to use violence as a way to express that anger.”